A conventional boiling water reactor is shown in FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. The feedwater from sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18.
Core shroud 18 is a stainless steel cylinder surrounding the nuclear fuel core 20. Core 20 is made up of a plurality of fuel bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1). Each array of fuel bundle assemblies is supported at the top by a top guide 19 and at the bottom by a core plate 21. The core top guide provides lateral support for the top of the fuel assemblies; the core plate provides lateral support for the bottom of the fuel assemblies. This lateral support maintains the correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 16 to the core lower plenum 24. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Vertical stand-pipes 30 atop shroud head 28 are in fluid communication with core upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 43 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 45. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 18.
The core shroud 18 (see FIG. 2) comprises a shroud head flange 18a for supporting the shroud head 28; a circular cylindrical upper shroud wall 18b having a top end welded to shroud head flange 18a; an annular top guide support ring 18c welded to the bottom end of upper shroud wall 18b; a circular cylindrical middle shroud wall comprising three sections 18d, 18e and 18f welded in series, with a top end of section 18d being welded to top guide support ring 18c; and an annular core plate support ring 18g welded to the bottom end of middle shroud wall section 18f and to the top end of a lower shroud wall 18h. The entire shroud is supported by a shroud support 50, which is welded to the bottom of lower shroud wall 18h, and by annular shroud support plate 52, which is welded at its inner diameter to shroud support 50 and at its outer diameter to RPV 10.
The core plate 21 is bolted to and supported by the support ring 18e. The resulting friction between core plate 21 and support ring 18e resists lateral deflection of the core plate relative to the support ring. However, during reactor operation high-temperature water underneath the core plate exerts an upward pressure. This pressure partly counteracts the force applied by the bolts which clamp core plate 21 to support ring 18e. This decrease in clamping force is accompanied by a decrease in the friction force at the joint formed by the core plate and support ring. As a result of this decrease in friction at the joint, the threshold force needed to cause relative lateral deflection of core plate 21 and support ring 18e is reduced.
In the event of a seismic disturbance, it is conceivable that the ground motion will be translated into lateral deflection of the fuel bundle assemblies 22 seated on the core plate 21 relative to the core shroud 18 and lateral deflection of the core shroud 18 relative to the reactor pressure vessel 10. If not adequately restrained, such deflections could cause unacceptable misalignment and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
The shroud can be restrained relative to the reactor pressure vessel using a repair which provides lateral and vertical restraint of shroud deflections. The repair uses spring supports which react seismic loads at localized points around the shroud circumference. For the shroud restraint repair to be effective in controlling core plate deflection, the concentrated restraint loads must be transmitted from the shroud to the core plate without causing sliding in the core plate to shroud joint. Friction in this bolted joint may not be adequate to resist sliding. Thus, there is a need for a device which can restrain the core plate against unacceptably large lateral displacements relative to the core shroud.